I took another field trip during my stay in the Bay Area: to Napa Valley. Unfortunately, I picked Labor Day weekend to do this, and everyone else in the Western Hemisphere was doing the same thing. Between the crowds and the fact that I was driving, I abstained from imbibing the local spirits.

But, like me, my day was not totally wasted, because I found a few geologically interesting places. The thing that most defines Napa Valley, besides the wine, is the volcanism. Back when there was subduction happening on the coast of California, there was a string of volcanoes along the continental margin, usually 50 to 200 miles from the subduction trench. This is always the case when you have subduction because that oceanic plate dives into the mantle, melts, and goes the only way it can go: up! This is still going on in the Cascades, as well as in Alaska, and Japan, and Chile…hey that’s pretty much the whole Ring of Fire thing, isn’t it?

One of the old volcanoes is Mt St Helena, because we don’t have enough volcanoes in this country with names like that.

It’s one of these, I think the one in the back. About 3.5 million years ago, one of these volcanoes erupted. And this was a pretty massive eruption. This was a felsic magma, which means it had a high silica content, which means it was very viscous and held onto gasses, which means that when it erupted, it was violently explosive. When this kind of lava cools quickly, you get rhyolite. In this picture, you can see little vesicles, indicating that there was still gas in this lava when it cooled.

This stuff is everywhere in the area around Calistoga. In fact, it’s this volcanic history that’s responsible for Calistoga’s famous hot springs. But we’ll get back to that in a little bit.

All this rhyolite buried a redwood forest. The wood tissue was eventually replaced by silicate minerals and the result is a petrified forest! It’s the only petrified forest in California and the only one to contain primarily petrified redwood trees.

Here’s a petrified redwood. It was about 2000 years old when the volcano blast knocked it down.

Here you can see how well the bark is preserved.

You can see the minerals that replaced the wood tissue. Some of the petrified trees are really quite lovely.

Here’s one that doesn’t really look like much from afar.

But up close, isn’t it beautiful?

This one is so large, they didn’t even excavate the entire thing. They just built a little mine shaft of sorts to extend back over 100 feet.

You can see how thick this layer of volcanic rock is over the tree.

Here are some more petrified trees:

That last one is called the “Robert Louis Stevenson” tree, and it does kind of look like a face with the shadows… Actually he stayed in the area for a time and wrote the book “Silverado Squatters” about his time there.

This is a beautiful piece of petrified wood in the fireplace of the museum/gift shop at the Petrified Forest.

The other cool thing I found in Calistoga was the “Old Faithful” geyser. Now I know what you’re thinking, Old Faithful is at Yellowstone, but apparently this name is something that any geyser can have as long it erupts fairly regularly. And this one was going off at 20 minute intervals while I was there, although it depends on how much groundwater there is which depends on how the precipitation has been. Here is the geyser pre-eruption.

What happens in a geyser is that the groundwater is sitting in cavities below ground, absorbing heat. What’s the source of the heat? A magma body. Now all of the volcanoes in the area are quite extinct, and haven’t erupted in millions of years, but there is still magma down there somewhere. In fact, there is at least one (maybe more) geothermal power plant in the area.

So the water is getting hot, but it doesn’t boil because it’s underground where the pressure is higher, so it gets superheated, meaning it’s above the boiling point. But hot water does expand, so the expanding water eventually fills all the little chambers until there’s no space left but to escape through the opening to the surface. This sudden release is like opening a shaken soda can. The weight of the water that escaped reduced the pressure on the water below it, causing the remaining water to instantly boil and erupt.

The eruption starts small with just some steam and a few burbles of water. But then it really gets going.

And if you have nothing better to do with 6 minutes of your life than watch water shoot out of the ground, here’s a video.

And if you do have nothing better to do, welcome to my sad little world.

But! That’s not all you get for your $10 at Old Faithful of California. I don’t know about you, but the first thing I think of when I think “geyser” is “fainting goats”, followed immediately by “Satanic freak-sheep”.

You’re not imagining things. That sheep does have four horns. It’s a Jacob sheep, and they can have up to six!

I swear this one was dreaming. It would occasionally lift its (her?) head just a bit and baaaa very softly but never open its eyes. It would also kick its legs rhythmically for a few seconds.

Last stop of the day was Clearlake, which is allegedly the largest lake in California.

I can’t really say how clear it is, but it smelled like a high density cattle feed lot. I don’t know why, but after standing next to it for a couple minutes, I started getting nauseous.

My guidebook said there would be obsidian visible in the roadcuts on the western side of the lake, and there were. Unfortunately there was no safe place to stop to take pictures. Other roadcuts south of the lake were serpentinite, but again, no safe place to stop. You’ll just have to take my word for it!

I’m going to leave you with one last photo, a hillside vineyard with an extinct volcano in the background, perfectly representative of the Napa Valley.